Currently, PDMs are tested using standardized ASTM protocols in order to simulate performance of parameters such as elastomer material performance and elastomer bonding performance under projected drilling loads. Conventional testing may be performed on failed PDM parts (typically stators) that have been retrieved from downhole service as part of an investigation into the cause of the failure. Alternatively, conventional testing may be performed as part of well planning activity, prior to drilling, in order to optimize selection of PDM components such as stator elastomer in view of the expected downhole environmental conditions and anticipated loading.
Conventional testing is done according to current applicable ASTM protocols. The ASTM tests are not particularly representative of specific expected or encountered downhole environments. First, the ASTM tests are not able to replicate the cyclic loading stresses encountered by PDMs in service. Second, the ASTM tests only peripherally simulate the elevated temperatures and pressures encountered by PDMs in service.
Generally, the ASTM tests are basic immersion tests in which an elastomer ample from a stator is first exposed to drilling fluid for a prescribed period at elevated temperatures, followed by performance evaluation by pull or tear testing. The performance evaluation is normally done at ambient temperatures and pressures. The purpose of the ASTM testing is to evaluate loss of physical properties in the rubber material and/or bond integrity. The ASTM testing may be done with a small volume of drilling fluid used in the specific drilling application. The drilling fluid sample may be taken from the actual drilling site.
Examples of specific conventional ASTM protocols include the following:                ASTM D2240—Durometer Hardness (Shore A)        ASTM D412—Tensile properties (Die C)        ASTM D5289—Vulcanization using Rotorless Cure Meter (MDR tests)        ASTM D624—Tear Strength (Die C)        ASTM D3182—Practice for preparing standard vulcanized sheets (for tensile and tear bars)        ASTM D429—Adhesion        ASTM D6370—Compositional Analysis by Thermogravimetry (TGA)        ASTM D471—Rubber Property—Effect of Liquids (Swell or immersion test)        
In addition to the traditional ASTM testing, two other conventional test methods are known to evaluate the performance of PDM power sections. In contrast to the traditional ASTM protocols described above, however, these two test methods require large volumes of test drilling fluid to perform each test.
Flow loop testing attempts to simulate downhole PDM service by placing a full downhole motor in an enclosed dynamometer (“dyno”). Drilling fluid is introduced at full operating pressure to drive the PDM power section. It often takes hundreds or even thousands of gallons of drilling fluid to fill and operate the flow loop. Each well drilled may have different drilling fluid types, making a full scale dyno test cost prohibitive to perform on a frequent basis. Further, the specialized equipment required to handle drilling fluids under pressure and high temperature make this approach logistically challenging and often not economically viable.
Recirculating pump vessels have also been used in the past to evaluate the performance of PDMs. While pump vessels do not require the volume of drilling fluid needed by flow loops, pump vessels still require a minimum of 25 gallons of drilling fluid to operate. Further, pump vessels have limited load ranges that correspondingly limit the range of downhole environments that can be simulated.
The volumes of drilling fluid needed by flow loop testing and recirculating pump vessel testing present a further drawback that is inherent to these conventional tests. Often the need for testing and evaluation arises after there has been a specific downhole PDM failure. Investigation of the failure advantageously includes evaluation and testing of the power section in an environment that closely simulates the downhole conditions in which the failure occurred. Such simulation is enhanced greatly when the evaluation and testing can be done using a sample of the actual drilling fluid flowing through the power section at the time the failure occurred. Such samples can be taken, and are available, but they come in small volumes, typically 0.5 to 2 gallons—not nearly enough to do flow loop or recirculating pump vessel testing.
There is therefore a need in the art for a test apparatus that can closely simulate, evaluate and measure PDM performance under (1) the elevated temperatures encountered downhole, and (2) the cyclic loads encountered downhole. The test apparatus should nonetheless provide embodiments in which only small quantities of drilling fluid are required, ideally less than about 10 gallons of drilling fluid, preferably less than about 5 gallons and most preferably, less than about 2 gallons. Embodiments of the test apparatus should also ideally have a small enough footprint to be suitable to be housed in a conventional laboratory or at a wellsite location.